An important criteria for customer satisfaction regarding a printing, copying, or multiple-function document device is the quality of stacking of the paper or other media that is output to a temporary or permanent storage location. For example, a laser printer provided with a remote output stacking unit may be operated to produce large stacks of multiple jobs, for example, wherein the jobs represent the same document printed out many times in succession, whether the document/job is a single sheet or many sheets. Such a stacking unit typically stacks the multiple jobs in a single stack, with the jobs contacting each other without any separating members or trays between the jobs as might be present between multiple copies in a photocopier sorter. The stacking unit may include means for allowing easy separation of the jobs, which may be conventional apparatus and logic to stack each job at a position slightly offset laterally from the previous job, that is, staggering the positions of alternating jobs a fraction of an inch to each side. Examples of conventional stacking technology, both for sheet alignment and lateral job offset, may be seen in several U.S. Pat. Nos.: 4,318,541 (Nagel et al.), 4,319,743 (Rood), 4,319,744 (Nagel et al.), 4,325,544, (Magno et al.), 4,325,545 (Fabrig), 4,354,672 (Kulpa et al.), 4,508,333 (Byrt), 4,548,399 (Heider et al.), 4,616,821 (Boeve et al.), 4,650,178 (Steele et al.), 4,657,239, (Ikesue et al.), 4,776,578 (Hirakawa et al.), 4,981,293 (Yamashita et al.), 4,986,730 (Wetter), 5,014,091 (Koike et al.), 5,054,764 (Phillips et al.), and 5,713,566 (Coombs et al.).
Stacker units of particular interest for implementation of the present invention are the type included on the 8000 Series LaserJet.TM. printers made by Hewlett Packard. Such a printer has a media output path that discharges paper or other media from the printer engine, with the paper edges being herein defined as a leading edge (first exiting the output path), the trailing edge (last exiting the output path), and two side edges of the paper which run transverse to the leading edge and trailing edge. The output path includes a device called an accumulator, which is typically external to the print engine housing and may be considered part of the stacking unit. The single sheet or plurality of sheets of an individual job being discharged are temporarily stored in the accumulator before being post-processed, if requested, and delivered to the final output destination. The accumulator comprises a floor, moveable sideguides with paper supports ("off-setters"), a fixed lower set of rollers, and a moveable set of upper rollers 51 that are used to align, and ultimately, to move the job. The off-setters of the accumulator contact and catch the two side edges of the accumulated sheets, and the floor of the accumulator supports the center region of the accumulated sheets. The successive sheets discharged into the accumulator 20 are inherently aligned with the other sheets in the accumulator. After a job is completed, that is, discharged in its entirely into the accumulator, the job is transported to the post-processing device, if requested. Then, the completed job is ejected by the accumulator onto a moveable stack tray disposed in front of and underneath the accumulator. The accumulator ejects the job, for example, by actuating a push-bar as in shown in the drawings, or by other systems, such as upper moveable rollers closing against lower fixed rollers and "pushing" the job to the "output bin." The output bin comprises the stack tray with any previous jobs that are stacked on top of the stack tray. Thus, if a job or jobs have been previously ejected onto the stack, tray, the current job is actually ejected onto the top of the previous job that, in turn, may be on top of prior jobs that rest on the stack tray. Prior to ejecting the current job onto the previous job, the off-setters of the accumulator are actuated, as determined by the logic of the system, to push the accumulated job to one side a predetermined amount of typically less than one inch before the job if ejected onto the stack. This way the various jobs are staggered or "offset" so that a first side edge of alternate jobs stick out laterally relative to the jobs above and below enough for a user to grasp and separate the jobs.
Typically, conventional media stacking technology results in misaligned sheets in some or all of the jobs, because there are no forward limits to control forward misalignment of sheets. The frequency of misaligned sheets may vary and may depend, for example, on the particular design of the stacking unit, maintenance of the stacking unit, paper quality, and ambient conditions such as humidity. The inventors believe that the majority of the stack misalignment is due to the force produced by ejecting the current job against the top sheets of the pre-existing stack as the current job slides forward and down from the accumulator to lie on the pre-existing stack. The ejected job slides against the output stack and displaces the top sheet or sheets from one to several inches, depending on the number of pages in the print job as well as the size and type of paper being printed.
Many stacking units, therefore, produce a misaligned stack that requires intermediate processing of many of the jobs prior to stapling, hole-punching, or binding. This intermediate processing involves separating the jobs and then straightening each misaligned job by pushing, tapping, or re-stacking the displaced sheets to align them with the rest of that job's sheets. This intermediate handling is time-consuming and frustrating for users, and adds significantly to the cost of document or booklet production.
Mechanisms have been added to some stacking units in attempts to correct the misalignment problem. For example, a rotating fingers mechanism has been added to the output device used with the Xerox Finisher for Xerox N32 and N40 printers, in such a way that the fingers hold down the top sheet in the output stack while the next job is ejected on the top of the output stack. After the job has been ejected, the fingers rotate out from under the ejected job and come to rest on top of the stack once again. The rotating finger mechanism has the disadvantage of requiring an additional, specialized mechanism to the accumulator/stacking unit, which increases costs and maintenance.
Therefore, an improved stacker is still needed, to reduce or eliminate manual processing of printed/copied jobs to correct displaced sheets. Still, there is a need for an economical, reliable solution to the displacement problem, whether the ejected jobs are a single sheet or multiple sheets, and the present invention addresses this need.